Low rank coals (LRCs) compose nearly 50% of the total coal produced in the United States and about one-third of the coal produced worldwide. A significant proportion of LRCs contain high levels of moisture that can prohibit utilization as feedstock for gasification and liquefaction, and other uses. A LRC with high-moisture content is not generally suitable for gasification because of low efficiency and poor yield of synthesis gas (a mixture of carbon monoxide and hydrogen). The high concentration of moisture is deleterious because of the extra energy required to evaporate moisture and its effect on the molar ratio of hydrogen and oxygen required for efficient production of synthesis gas or the subsequent production of naphtha and diesel fuel by the Fischer-Tropsch process. The production of synthetic natural gas and liquefaction products is also negatively impacted by excess moisture.
Some gasifiers accept solid feedstock, and others accept slurry feed. The slurry-feed type gasifiers have been used for many years and are well suited to gasify coal provided that the slurry contains a specified minimum energy content, typically 16,282 kJ/kg (7,000 Btu/lb). Currently, slurry-fed gasifiers use fuel feedstocks with a low inherent moisture such as high-grade coals or coke. These fuel feedstocks produce slurries with a sufficiently high thermal energy content to meet the economic requirements of gasification.
Availability of high-grade coal and excessive transportation cost has, to date, excluded large areas of the western United States from consideration as viable locations for gasification. LRC's in these regions are abundant, but have too much inherent moisture to make high-energy content slurry.
In low rank coals, water is present in two forms: inherent moisture, a form that is contained wholly within the particle, and surface moisture, present on the exterior of the particle as a liquid. The water contained in the particle does not contribute to forming slurry, but only serves to dilute the energy content contained in the coal. Water that is exterior to the particle provides the vehicle to form a slurry. Sufficient water, typically 30 wt %, is required to provide a slurry mixture that has the desired rheological properties to be stored, mixed, pumped, and fed into the gasifier. The sum of inherent water and exterior water present in LRCs dilute the energy provided by the coal solids to a degree that it is not acceptable as an economic feed to the gasifier.
Researchers have attempted to reduce inherent moisture in an attempt to form a high-energy slurry. Various methods have been proposed to treat the raw LRC prior to slurry production. They include thermal drying with direct contact of high-temperature gas, indirect drying with contact of the coal with high temperature surfaces, and hydrothermal treatment where coal is immersed in a water-filled vessel held at high temperature and pressure. These methods have proved unsatisfactory due to excessive cost or production of a product that will re-absorb moisture when immersed in liquid water during the slurry production process.
Researchers also have attempted to identify agents that can be applied to the dried material to render the product incapable of reabsorbing moisture when immersed in water. Water-repellent chemicals such as waxes and petroleum-based materials have been used. Unfortunately, the cost of chemical treatment has largely precluded the use of these chemicals.
Thus, there remains a need for effective and cost-efficient processes to prepare low rank coals for use in gasification and liquefaction.